Culex pipiens complex distribution in Mexico City. Map of Cx. p. quinquefasciatus, Cx. p. pipiens and hybrid distribution in all collection sites according to nucleotide differences in the Ace.2 gene. Pie chart graphs (numbers in black) indicate the distributional frequency of identified lineages. The sizes of the pie chart's segments are proportional to the number of mosquitoes identified as Cx. p. quinquefasciatus (blue), Cx. p. pipiens (red) or hybrid (green). The numbers in blue indicate the sites where Cx. p. quinquefasciatus was exclusively identified, red where Cx. p. pipiens was recognized and in black where hybrids were localized according to chart graphs. In addition the number in brown indicates the site where Cx. p. quinquefasciatus and Cx. tarsalis was observed. The grey line approximates the probable hypothetical introgression area. The numbers correspond to the collection sites displayed in the Additional File 1.